The present invention relates to laminators of the type used to laminate a sheet such as a photograph, certificate or chart for example between a pair of transparent plastics sheets. More particularly, although not exclusively, the invention relates to a laminator having a mechanism for detecting the thickness of the layers to be laminated and means for adjusting the through-speed of the sheets past the laminator""s heating rollers depending upon the detected thickness.
Known laminators comprise a pair of internally heated rollers that are biased toward one another or at a fixed spacing and in between which the layers to be laminated are pressed. The heat from the rollers melts an adhesive surface on each of the plastics sheets and the melted laminate infuses to some extent into the sheet that is being laminated. Cards and thin paper are relatively straightforward to laminate because a pre-set temperature and speed setting of the rollers achieves the necessary adhesion between the layers. However, when a thick sheet of cardboard for example is to be laminated, the pre-set temperature and speed setting can result in insufficient adhesive penetration to the cardboard, and consequent ease of delamination of the finished product. For this reason, the laminated finished product might have to be passed a second time through the machine and perhaps even a third. This can result in loss of productivity or even damage to the finished product.
It is an object of the present invention to overcome or substantially ameliorate the above disadvantage and/or more generally to provide a thickness sensor for a lamination machine and a lamination machine incorporating a thickness sensor and speed-controlled lamination/heating rollers.
There is disclosed herein a thickness sensor for layered sheet material entering a lamination machine, comprising:
an arm movable upon interaction with the layered sheet material, and
means providing variable resistance output dependent upon the position of the arm.
Preferably the means providing variable resistance comprises an electrical conductor mounted to the arm and providing a pair of exposed electrical contacts, and a printed circuit board (PCB) having exposed tracks engaged by respective ones of the electrical contacts, and resistors electrically connecting adjacent portions of at least one of the tracks.
Preferably the tracks each comprise an end portion electrically isolated from one another and engaged by respective ones of the electrical contacts for indicating that the lamination is too thick.
Preferably one of said end portions comprises a sub portion electrically isolated therefrom and engaged by one of the electrical contacts for providing a warning signal that the lamination is of maximum allowable thickness.
Preferably the arm is pivotally mounted upon a pivot pin and the tracks are curved.
Preferably the arm comprises a roller at one end engagable with the layered sheet material and the pivot pin is closer to the roller than it is to the electrical contacts.
There is further disclosed herein a lamination machine comprising:
a pair of heated rollers between which layers of sheet material are pressed for lamination, and
the above-disclosed thickness sensor attached to the machine at a location upstream of the rollers and cooperating with the machine to vary the speed of rotation of the rollers.
Preferably at least one of said rollers is driven by an electric motor, the operational speed of which is affected by the thickness sensor.
Preferably the machine further comprises a pair of withdrawal rollers downstream of the heated rollers.
Preferably the withdrawal rollers rotate in unison with the heated rollers via a gear train therebetween.